This invention relates to an inspection apparatus and inspection method using an electron beam for inspecting a semiconductor device, a substrate, a photomask (exposure mask), a liquid crystal, etc. having a fine pattern
The semiconductor devices such as the memory and the microcomputer used for the computer are fabricated by repeating the steps of exposure, lithography, etching, etc. on a pattern such as a circuit formed on a photomask. In the fabrication process of the semiconductor device, the success or failure of the result of the steps such as lithography and etching and the presence of a defect such as foreign matter have a great effect on the production yield of the semiconductor device. In order to detect abnormalities or defects at an early time or in advance, the pattern on the semiconductor wafer is inspected at the end of each fabrication step.
According as the diameter of the wafer increases and the circuit pattern is micronized, an image of a high SN ratio is required to be acquired very rapidly to conduct an inspection with a high throughput and a high accuracy. For this purpose, the number of electrons radiated and a high SN ratio are secured by using a large current beam not less than 1000 times (not less than 100 nA) that for the normal scanning electron microscope (SEM). Further, it is essential to detect the secondary electrons and the reflected electrons from the substrate at high speed and with high efficiency.
Also, in order to keep the semiconductor substrate having an insulating film such as a resist free from the effect of the charge, a low-acceleration electron beam of not more than 2 keV is radiated. This technique is described in “ELECTRON • BEAM HANDBOOK” edited by 132 Committee of Japan Society for the Promotion of Science (Nikkan Kogyo Shimbun, Ltd., 1986), Pages 622-623. An electron beam with a large current and a low acceleration, however, causes the aberration due to the space-charge effect and makes the observation with high resolution difficult.
For solving this problem, a method is known in which the high-acceleration electron beam is decelerated immediately before a sample and radiated on the sample as a substantially low-acceleration electron beam. This technique is described, for example, in JP-A-2-142045 and JP-A-6-139985.
JP-A-2003-83917, on the other hand, describes an appearance inspection apparatus using an electron beam, comprising a pixel pitch determining means for determining the pixel pitch along the direction in which pixels making up an image are scanned by the electron beam or a line pitch determining means for determining the line pitch, i.e. the pixel pitch along the direction in which the electron beam is fed, wherein the inspection rate is determined by an inspection rate determining means based on the pixel pitch or the line pitch determined by the pixel pitch determining means or the line pitch determining means, respectively.
The inspection apparatus using the SEM without any optical system poses the problem described below.
In the inspection apparatus of SEM type, the electron beam is scanned on each line in one stroke, and therefore, the throughput is reduced as compared with the optical inspection apparatus which can cover the whole line at a time. This tends to become more conspicuous with the reduction in defect size due to the micronization of the device design rule. This tendency poses another problem even with a defect large in size.
In the case where a defect is small in size, the detection pixel pitch is required to be reduced in accordance with the defect size and the area capable of being inspected in unit time is reduced for a lower throughput. To prevent this inconvenience, the number of pixels making up each line is increased to compensate for the reduced area.
In the case where a defect is large in size, on the other hand, an increase in detection pixel pitch in accordance with the defect size may excessively increase the scanning width determined by the product of the pixel pitch and the number of pixels for the number of pixels increased as in the aforementioned case of small defect size, and the optoelectronic limit may be exceeded, thereby reducing the quality of the peripheral image. As a result, many imaginary defects occur due to the image distortion along the periphery of the image, resulting in an extremely reduced defect detection performance.